A steam turbine using primary steam having a temperature of approximately 600° C. is in practical use from the viewpoint of improvement in turbine efficiency. To further improve the turbine efficiency, studies on increasing the temperature of the primary steam to a value ranging from approximately 650 to 750° C. have been conducted and developments according to the studies have been performed.
In such a steam turbine, since the primary steam is of high temperature, it is necessary to use a heat-resistant alloy as in the case of a gas turbine. However, no heat-resistant alloy can be used, for example, because such a heat-resistant alloy is expensive and makes it difficult to manufacture a large component. In such case, the strength of the material of the turbine is insufficient and it is necessary to cool the components of the turbine.
Japanese Patent Laid-Open Publication No. 11-200801 (Patent Document 1) discloses a cooling mechanism used with rotor discs integrated with a rotor and studded with blades. The cooling mechanism cools the vicinity of blade studded portions of the rotor discs, in particular, rotor discs in the second state and the following stages. In the cooling mechanism, a cooling fluid is directly supplied into cooling spaces formed by side surfaces of the rotor discs and internal side surfaces of vanes through cooling path holes formed in the rotor.
However, it is not easy to readily form the cooling path holes, which are provided to cool the vicinity of the blade studded portions of the rotor discs as described in Patent Document 1, in the rotor inside the rotor discs, and it is also not always preferred to form the cooling path holes from the viewpoint of ensuring the strength of the rotor.
Further, in turbine stages that require cooling, such as the rotor discs, the cooling steam that contributed to the cooling in the upstream side turbine stages and then cools the cooling steam increased in temperature in the downstream side turbine stages, which may cause a case of insufficient cooling.